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Chapter 1: Data Flow DiagramStructuring System Process RequirementsChapter 7 in Modern System Analysis and Design Book.

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Overview

Process Modeling and Data Flow Diagrams (DFDs).

Draw DFDs of well structured process models.

Decompose DFDs into lower-level diagrams.

Balance high-level and low-level DFDs.

The differences between current physical, current logical, new physical, and

new logical DFDs.

Using DFDs for analyzing information systems.

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System Development Life Cycle “SDLC”

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Process Modeling

A technique for graphically representing the processes that are used to capture,

manipulate, store, and distribute data;

between a system and its environment,

among system components.

Build a DFD using information gathered during requirements gathering and

determination.

Both processes and data structures are modeled in DFDs.

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Process ModelingDeliverables and Outcomes

Context data flow diagram (DFD).

Shows the scope of a system (i.e., a top-level view).

Often DFDs are created showing the current physical and logical system.

It enables analysts to understand how the current system operates.

DFDs of new logical system.

The DFD is independent of technology.

It shows data flows, structure, and functional requirements of the new system.

Includes a thorough description of each DFD component.

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Data Flow Diagram (DFD)

A picture of the movement of data between external entities and the processes and

data stores within a system.

How does a DFD differ from a systems flowchart?

DFDs depict logical data flow independent of technology.

The focus is on data flows, not process flows alone.

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Comparison between DFD Symbols Sets

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DFD Symbols

Process: work or actions performed on data (inside the system).

Data Store: data at rest (inside the system).

Source/Sink: external entity that is origin or destination of data (outside the system).

Data flow: arrows depicting movement of data.

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DFD Diagramming Rules Process

No process can have only outputs or only inputs. Processes must have both outputs and inputs.

Process labels should be verb phrases.

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DFD Diagramming Rules Data Store

Data Store labels should be noun phrases.

All flows to or from a data store must move through a process.

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DFD Diagramming Rules Source/Sink

Source and Sink labels should be noun phrases.

No data moves directly between external entities without going through a process.

Interactions between external entities without intervening processes are outside the system and therefore not represented in the DFD.

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Bidirectional flow between process and data store is represented by two separate arrows.

Forked data flow must refer to exact same data item (not different data items) from a common location to multiple destinations.

DFD Diagramming Rules Data Flow

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Joined data flow must refer to exact same data item (not different data items) from multiple sources to a common location.

Data flow cannot go directly from a process to itself, must go through intervening processes.

DFD Diagramming Rules Data Flow

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DFD Diagramming Rules Data Flow

Data flow from a process to a data store means update (insert, delete or change).

Data flow from a data store to a process means retrieve or use.

Data flow labels should be noun phrases.

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Functional Decomposition

An iterative process of breaking a system description down into finer and finer detail.

High-level processes described in terms of lower-level sub-processes.

DFD charts created for each level of detail.

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DFD Levels

Context DFD

Overview of the organizational system.

Level-0 DFD

Representation of system’s major processes at high level of abstraction.

Level-1 DFD

Results from decomposition of Level 0 diagram.

Level-n DFD

Results from decomposition of Level n-1 diagram.

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Context Diagram of Hoosier Burger’s food ordering system

Context diagram shows the

system boundaries, external

entities that interact with

the system, and major

information flows between

entities and the system.

NOTE: only one process symbol, and no data stores shown.

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Level-0 DFD

Level-0 DFD shows the

system’s major

processes, data flows,

and data stores at a high

level of abstraction.

Processes are labeled

1.0, 2.0, etc. These will

be decomposed into

more primitive (lower-

level) DFDs.

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Level-1 DFD

Level-1 DFD shows the

sub-processes of one of

the processes in the Level-

0 DFD.

This is a Level-1 DFD for

Process 4.0.

Processes are labeled 4.1,

4.2, etc. These can be

further decomposed in

more primitive (lower-level)

DFDs if necessary.

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Level-n DFD

Level-n DFD shows the sub-

processes of one of the

processes in the Level n-1 DFD.

This is a Level-2 DFD for

Process 4.3.

Processes are labeled 4.3.1,

4.3.2, etc. If this is the lowest

level of the hierarchy, it is

called a primitive DFD.

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DFD Balancing

The conservation of inputs and outputs to a data flow process when that process is

decomposed to a lower level.

Balanced means:

Number of inputs to lower level DFD equals number of inputs to associated processof higher-level DFD.

Number of outputs to lower level DFD equals number of outputs to associatedprocess of higher-level DFD.

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Unbalanced DFD

This is unbalanced because

the process of the context

diagram has only one input

but the Level-0 diagram has

two inputs.

Context Diagram

Level-0 Diagram

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Balanced DFD

These are balanced because the

numbers of inputs and outputs of

context diagram process equal the

number of inputs and outputs of

Level-0 diagram.

3 outputs

1 input

Context Diagram

Level-0 Diagram

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Balanced DFD

These are balanced because the numbers of

inputs and outputs to Process 1.0 of the

Level-0 diagram equals the number of

inputs and outputs to the Level-1 diagram.

1 input

4 outputs

Level-0 Diagram

Level-1 Diagram

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Data Flow Splitting

A composite data flow at a

higher level may be split if

different parts go to

different processes in the

lower level DFD.

This remains balanced because the same data is involved, but split into two parts.

Complex data flow

Disaggregated data flows

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More DFD Rules

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Four Different Types of DFDs

Current Physical

Process labels identify technology (people or systems) used to process the data.

Data flows and data stores identify actual name of the physical media.

Current Logical

Physical aspects of system are removed as much as possible.

Current system is reduced to data and processes that transform them.

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Four Different Types of DFDs

New Logical

Includes additional functions.

Obsolete functions are removed.

Inefficient data flows are reorganized.

New Physical

Represents the physical implementation of the new system.

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Physical vs. Logical

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Guidelines for Drawing DFDs

Completeness

DFD must include all components necessary for system.

Each component must be fully described in the project dictionary or CASE repository.

Consistency

The extent to which information contained on one level of a set of nested DFDs is also

included on other levels.

Timing

Time is not represented well on DFDs.

Best to draw DFDs as if the system has never started and will never stop.

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Guidelines for Drawing DFDs

Iterative Development

Analyst should expect to redraw diagram several times before reaching the closest

approximation to the system being modeled.

Primitive DFDs

Lowest logical level of decomposition.

Decision has to be made when to stop decomposition.

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Guidelines for Drawing DFDs

Rules for stopping decomposition

When each process has been reduced to a single decision, calculation or database

operation.

When each data store represents data about a single entity.

When the system user does not care to see any more detail.

When every data flow does not need to be split further to show that data are handled in

various ways.

When you believe that you have shown each business form or transaction, online display

and report as a single data flow.

When you believe that there is a separate process for each choice on all lowest-level menu

options.

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Using DFDs as Analysis Tools

Gap Analysis

The process of discovering discrepancies between two or more sets of data flow

diagrams or discrepancies within a single DFD.

Inefficiencies in a system can often be identified through DFDs.

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Using DFDs as Business Process Reengineering

Before: Credit approval process required six days.

After: Process 100 times as many transactions in the same time.